The feasibility of multi-color self filtering, infrared detectors based on epitaxial lead salt thin film technology has been demonstrated in the prior art. The multicolor response capable of detecting infrared radiation at certain selective wave bands is made possible by the growth of successive epilayers of lead chalcogenide or its alloys with tin or cadmium on a single barium fluoride substrate. Each layer gives photoresponse to infrared radiation of wave lengths or the corresponding frequencies, characteristic of the layer chemical composition, while also serving as a photon filter for the succeeding layers i.e. each semiconductor layer is used as a detection layer and as a filter layer. Since the wave band if each layer is determined by the chemical composition of the layer, it can therefore be "tuned" to some predetermined interval. The spectral range of wave lengths from 2 to 15 microns can thus be covered in this invention.
In the prior art incorporation of this multi-color detector technology the multilayers are grown successively each succeeding layer on top of the previous layer. Non-ohmic contacts and ohmic contacts are deposited on the top of the single exposed surface of each of the semiconductor layers. Typical and illustrative of this technology is the U.S. Patent to Campbell 4,323,911. In this patent a number of epilayers of semiconductor materials are successively coated one on another with the top layer being fitted with an ohmic contact and a non-ohmic contact.
It is well established in the prior art that single crystal films of lead selenide (PbSe), lead sulfide (PbS) and related compounds hereinafter referred to as lead salt alloys can be expitaxially grown on heated alkali halide substrates by vacuum evaporation.
The state of the art progressed as taught in U.S. Pat. No. 154,631 entitled "Equilibrium Growth Technique For Preparing PbS.sub.x Se.sub.1 - x, Epilayers" , filed in May 1977 by R. B. Schoolar. This patent disclosed and claimed a novel process wherein a single lead sulfide selenide epilayer, having predetermined electrical and optical properties was prepared by an equilibrium growth technique (EGT).
An alkali halide substrate maintained in near thermodynamic equilibrium with source charges is exposed to the single chimney orifice of a two zone dual chamber furnace in which a homogeneous vapor has been produced by sublimation of a lead chalcogenide alloy in one chamber and measured amounts of chalcogen in an adjacent chamber.
In a subsequent progression in the prior art, as disclosed and claimed in U.S. Pat. No. 4,282,045 to Schoolar et al. a modification of the equilibrium growth technique (EGT) was disclosed and claimed. In this patent the step of varying the alloy composition of a lead chalcogenide layer during its growth by changing the temperature of the heated substrate was a step forward in the art.
Another U.S. Pat. No. 4,282,045 issued to Dr. James D. Jensen and Richard Schoolar disclosed a variable temperature method for the preparation of multiple epitaxial layers of certain lead chalcogenide alloys wherein during preparation the temperature of the substrate was varied, providing a epilayer with graded composition and predetermined electrical and optical properties along the direction of growth. This growth technique was used to produce infrared lenses, narrow band detectors and double heterojunction lasers. In all of these prior art patents and processes known to the applicant, only one epitaxial layer was considered to be the top layer or upper layer and such layer was fitted and equipped with one non-ohmic contact, or the filter layer itself is made up of a multiplicity of semiconductor material layers, thereby limiting the spectral response capabilities of an infrared detector.